For example, a floating gate NAND flash memory which is a nonvolatile semiconductor memory device has a memory cell of a stack gate structure in which a gate insulating film (tunnel insulating film), a floating gate electrode, a gate insulating film (intergate insulating film), and a control gate electrode are stacked. In this memory cell structure, the upper part of the floating gate electrode becomes thinner along with the advance of generations, and the concentration of an electric field in this upper part is increased, so that a leakage current in the intergate insulating film is increased during writing.
Therefore, when the above-mentioned memory cell is miniaturized with no change of the conventional structure, electrons injected into the floating gate electrode escape toward the control gate electrode because of the increased leakage current even though a high write voltage is applied to increase a tunnel current passing through the tunnel insulating film and thereby increase the amount of electrons injected into the floating gate electrode. Consequently, a multivalued memory cell may reach write saturation that prevents writing up to a threshold necessary for multivalue writing.